Great question. One, I think, that many Christians either never think to ask, or are afraid to ask. They want as much latitude as possible, it appears to me, to shoehorn into Christian living, into life in the Spirit, a great deal of what is carnal. It doesn't help that secular society is mad with hyper-subjectivity and the insane resistance to objective - and constraining - reality. I think this hyper-individualism and subjectification of Truth has crept into the Church, manifesting in things like "Progressive Christianity" and the way so many churches tribalize within themselves into age groups, marital status groups, interest groups, political groups, and so on.
Anyway, how does one distinguish the illumination of the Spirit from a counterfeit of the same? I turn to the Bible for answers, of course, considering the matter along the following lines:
What does the Bible tell me the flesh looks like and is ultimately aimed at?
What does the Bible tell me the life and work of the Spirit looks like and is ultimately aimed at?
When these things are clearly-defined in one's thinking, is it possible to distinguish clearly fleshly Self-talk from the illuminating work of the Spirit?
The flesh (by which I mean the "carnal mind" - Romans 8:5-8) is well-described in the NT. It's temporally-focused, slave to the appetites and impulses of the body, radically seeking its own satisfaction and gratification (Philippians 3:18-19). Such selfishness always produces conflict with others, especially with those who are likewise selfish (1 Corinthians 3:1-3; James 3:14-16; Titus 3:3). Such self-centeredness must inevitably be sensual, desiring stimulation of the physical senses and emotions, leading often to various addictions (porn, drugs, food, etc. - 2 Peter 2; Jude 1:4-16).
The life of the Spirit stands in very sharp contrast to the life carnal living produces (even when it is cloaked in religious piety and external law-keeping). Rather than being Self-focused, the Spirit is always glorifying Christ, pressing the child of God to look away from themselves and grow more and more occupied with Jesus (John 16:14; Philippians 1:21; Philippians 3:7-8; 2 Corinthians 3:18, 1 Corinthians 2:2, etc.). The fruit of the Spirit is never more of us, but more of himself, who is the Spirit of Christ (Romans 8:9; Galatians 5:22-23) producing ever-increasing holiness, self-sacrifice, and truth in the life of the believer, always fanning the flames of desire for Christ and resisting our carnality.
So, when I think I've been "illuminated" by the Spirit, I do an analysis, putting the "new understanding" I think I've had under the "magnifying glass" of what I've pointed out from God's word. In what direction does my "lightbulb moment" move me? Toward self-congratulation and pride, or toward humility and an increased appreciation for God and His awesome wisdom and Truth? Does my new understanding bring God more into focus, or myself? As a result of the Spirit's illumination, am I left pondering my own psychology and intellect, my own insightfulness, my own life-story, or am I delighted by the knowledge that, having learned more of God, I move closer to Him and am better able to be used by Him?
Does what I want to say is from the Spirit, his illumination, correspond precisely with God's Truth revealed in Scripture? In immediate context, is what I think is Spirit- illumination of God's Truth correct? Does what I want to call the Spirit's teaching synthesize well with the rest of God's word, also? Does this illumination conform to God's laws of logic and principles of reason?
Finally, is what I think that the Spirit's taught me impacting my practical living and my own thinking, values and beliefs such that Christ is more evident in me (Romans 8:29; 2 Corinthians 4:7-11)? Most importantly, in consequence of the Spirit's illumination, do I desire God more (Matthew 22;36-38)? Is my love for Him increased, deepened and intensified, such that I will do anything to protect my communion with Him, first and foremost, to die to myself (Matthew 16:24-25; John 12:24-25; Colossians 3:1-3)?
When it's just carnal Self counterfeiting the Spirit's illumination, Christ, love for him, fellowship with God and the crucified life are not radically front-and-center. Instead, sensuality (not sexuality, necessarily) and emotionality, self-improvement, self-analysis, and self-interest supplant the Spirit's Christ-centeredness. Self-serving philosophies of the World often mix into my what I want to say the Spirit's taught me, too, watering down God's Truth, even contradicting it sometimes, making lots of room for pious carnality.
See above. I won't ever point to some inner, subjective "knowing" in justification of my claim that my "lightbulb" moment is from/of God. If I can't reconcile well my illumination by the Spirit with God's Truth given to me in His word, then I will not say my Spirit-illumination is of the Spirit. If the Spirit's illumination doesn't make me more like Christ, inflame my desire (love) for God, and prompt me to ever-more radical heart-separation from the World, the Flesh and the devil, then I'm lying (primarily to myself) if I say it has been the Spirit who has illuminated me.
Well, as you can see above, there's bit more to it than this.
I'm not clear on what you mean here...
Impossible, I would say.
Right. Which it's so incredibly important for Christians to be able to objectively analyze the actual character and source of their thoughts.
Yes, absolutely.
Yup.
That this bit is entirely subjectivized, determined to be so almost entirely on a "I just know" basis that has allowed a great deal of blasphemous stuff to go on among Christians. In consequence of this radical subjectivity about life in the Spirit, religious shysters, who are always keen to make merchandise of believers, offer weekend conferences where, for hundreds of dollars a pop, one can learn to hear the incredibly still and small "voice" of the Spirit, thrash about on the floor in a fit of hysteria, soak, or toke, or be drunk in the Spirit, or practice growing their faith by attempting to run through a wall. Awful, degrading and deeply carnal (as well as demonic) stuff.
No, I'm opposed to those who would offer a very unbiblical and highly-subjective counterfeit of life in the Spirit doing so. I've seen terrible devastation spiritually among those who have been chewed up and spat out by the hyper-charismatic culture, all the result of a migration away from the objective standard of God's word.